I carry a small pocketknife most of the time. It's a profoundly useful tool: In the office, I use it to open letters and packages. On construction jobsites, I use the screwdriver blades to open control panels and make adjustments to potentiometers and small wires. I've used the tweezers to remove wooden splinters from my thumb, and the corkscrew and bottle openers to keep good times going.

But fundamentally, the pocketknife is most valuable for the knife itself. A small, sharp blade is useful practically every day, several times a day. Of course, there's always the risk that someone could use it as a weapon -- which is why not a single sharp object is supposed to be allowed on passenger flights in America. But the danger is really in the hands of the person carrying it -- an evil person out to do harm will do so with whatever object he or she can find. As a tool, the knife is neutral. It's a piece of technology -- albeit one that has been around for two million years -- but its use for good or for bad is in the hands of the user.

The good user, though, will find nothing of value in a dull knife, and a knife will always go dull with use. So it must be sharpened. A sharp knife is safer than a dull knife, because it's less likely to slip or behave unpredictably in the hands of the user.

There's a certain imperative to living like a "sharp knife":

■ Each of us has an obligation to be useful. The world only moves ahead if we all participate. There are plenty of temptations to stand aside when we're needed or to sit down and do nothing, because that's easy. But what use is a sharp knife that sits in a drawer, growing rusty through neglect? It's just occupying space that could be occupied instead by something productive.

■ Each of us has an obligation to be used for good. The knife is an inanimate object; it doesn't know for what purpose it's being used. But we're animate -- better yet, we're sentient -- so we know what we're doing. And though we may not always know what forces or powers are ultimately behind what we do (or what we're told to do), we have the ability to try to find out. Because we can, we should. Just as we would take weapons out of the hands of evil people if we could, we should take ourselves out of the doing of evil whenever we can.

■ Each of us has an obligation to become sharpened. The knife that sits unused in the drawer will simply grow rusty and fall into disrepair. But the used knife will grow dull -- especially when used for the same purposes over and over -- and it has to be sharpened to retain its peak utility. Life uses us up. It beats some down more than others, but it weathers us all. And if we don't routinely find ways to sharpen ourselves mentally, then we eventually grow dull. People find all kinds of excuses for growing dull -- "Old dogs don't learn new tricks", for instance -- but if we let our faculties fade just because we're used to being used a certain way, or because sharpening ourselves seems too hard, then we're not much more useful than the rusty knife in the drawer.

Sharpening mental capacities and skills is essential, just like sharpening a knife. It's a natural part of being useful, and of being used for good purposes. Just like sharpening a knife on a stone, it can seem tedious -- but the payoff is that a good sharpening makes many future uses possible. It's too easy to grow dull or rusty. But in the sharpening is all of the utility.